X-ray & Optical Images of Comets ISON & PanSTARRS
Astronomers used Chandra to observe Comet ISON and Comet PanSTARRS in 2013, when these comets were relatively close to the Earth. These images show the comets in optical light as taken by an astrophotographer, with insets showing the X-ray data from Chandra. The X-ray emission is produced when a wind of particles from the Sun - the solar wind - strikes the comet's atmosphere. The Chandra data was used to estimate the composition of the solar wind, including the amount of carbon and nitrogen, finding values that agree with independent measurements.
(Credit: X-ray: NASA/CXC/Univ. of CT/B.Snios et al, Optical: DSS, Damian Peach (damianpeach.com))